1. Field of Invention
This invention relates generally/to the read/write heads used to generate and analyze the radiation beam which interacts with the storage medium in optical information storage and retrieval units. In particular, a bi-directional grating coupler is used to couple to the radiation in such a manner as to provide, when processed, tracking control and focus control signals.
2. Description of the Related Art
Referring now to FIG. 1, a typical read write head used in the interaction of an optical information and retrieval system with a storage medium is shown. Radiation source/collimator 11 provides a collimated beam of radiation. The radiation beam is applied to polarizing beam splitter 12 which provides a substantially linearly polarized beam of radiation. The polarized beam of radiation is passed through quarter wave plate 13, and focused on the storage medium 15 by lens 14. The radiation reflected from the storage medium is recollimated by lens 14 and the recollimated radiation is applied to the polarizing beam splitter 12. The linearly polarized radiation beam, having been rotated 90.degree. by the double passage through the quarter wave plate, is reflected by polarizing beam splitter 12 and applied to beam splitter 16. The radiation beam passing through the beam splitter 16 is focussed by lens 18 on detector 19. Detector 19 has stops 19A and 19B associated therewith which can provide the signals which can be used for information retrieval and for tracking control. The light reflected from beam splitter 16 is passed through lens 17 and applied to detector 18. Detector 18 includes stops 18A and 18B which provide a signal for controlling the focussing of the radiation beam on the storage medium. As will be clear to those skilled in the art, the detector relating to tracking control and the detector related to related focus control provide signals which, after appropriate processing, control servo-motors determining the radial position of the radiation focused on the disk and determining the distance of the focusing lens from the disk storage medium. In addition, at least one detector generates an information bearing signal which can be processed to identify the data stored on the optical storage disk in the form of variations in optical properties.
However, in spite of the success of the optical read/write heads, a need has been felt for apparatus and a method for providing a functionality equivalent to that found in a typical optical read/write head such as is shown in FIG. 1 while decreasing the bulk and weight of the read/write head. Recently, the use of holographic optical elements and diffraction gratings have been studied in a attempt to replace the optical components which are currently used to implement the read/write head. These newly considered elements can result in higher performance read/write optical heads with lower access times and reduced cost. A need has been felt for a read/write head configuration that effectively incorporates these newly considered elements.